Titanium metal has revealed its attractive properties one after another and it has been put to commercial use not only in the aircraft and spacecraft industries for many years but also in consumer goods such as cameras, glasses, watches and golf clubs in recent years; still more, titanium metal is expected to create a demand in the industrial sectors of construction materials and automobiles.
At the present time, the only method available for the commercial production of titanium metal is the so-called Kroll process with the exception of an electrolytic process employed on an extremely small scale for the production of high-purity titanium for use in semiconductors.
The smelting of titanium metal by the Kroll process is performed in the manner shown in FIG. 13.
In the first stage (S1), the raw material titanium oxide (TiO2) is allowed to react with chlorine gas (Cl2) at 1000° C. in the presence of carbon (C) to give titanium tetrachloride (TiCl4) with a low boiling point of 136° C. (chlorination: S101) and the titanium tetrachloride thus obtained is refined by distillation thereby removing impurities such as iron (Fe), aluminum (Al) and vanadium (V) and raising the purity of titanium tetrachloride (refining by distillation: S102); the formation of titanium tetrachloride involves the following reactions;TiO2+C+2Cl2=TiCl4+CO2TiO2+2C+2Cl2=TiCl4+2CO
In the second stage (S2), titanium tetrachloride is reduced to titanium metal in the presence of magnesium metal (Mg) (reduction: S201). The reduction is conducted by introducing magnesium metal to a hermetically sealed iron vessel, melting the magnesium metal at 975° C. and adding titanium tetrachloride in drops to the molten magnesium metal. Titanium metal forms according to the following reaction formula:TiCl4+2 Mg=Ti+2MgCl2
The titanium metal obtained by the reduction of titanium tetrachloride normally occurs as a large lump reproducing the inner shape of the apparatus used for the reduction reaction, for example, as a cylindrical lump; it is a porous solid or the so-called titanium metal sponge and contains the byproduct magnesium chloride and the unreacted magnesium metal; generally, the center of the sponge has dissolved oxygen on the order of 400-600 ppm and is tough while the skin has dissolved oxygen on the order of 800-1000 ppm and is hard.
This titanium metal sponge is then subjected to vacuum separation where the sponge is heated at 1000° C. or above under reduced pressure of 10−1-10−4 Torr to separate the byproduct magnesium chloride (MgCl2) and the unreacted magnesium metal (vacuum separation: S202).
The magnesium chloride thus recovered by the vacuum separation is decomposed by electrolysis into magnesium metal and chlorine gas (Cl2) (electrolysis: S203), the magnesium metal recovered here is utilized, together with the unreacted magnesium metal recovered earlier in the vacuum separation (not shown), in the aforementioned reduction of titanium tetrachloride while the recovered chlorine gas is utilized in the aforementioned chlorination of titanium oxide.
In the third stage (S3) where this titanium metal sponge is converted into the product titanium ingot by the consumable-electrode arc melting method, the sponge formed as a large lump is crushed and ground (crushing and grinding treatment) in advance for the preparation of primary electrode briquettes. If circumstances require, the ground sponge is sorted out in consideration of the purpose of use of ingot and the difference in the concentration of dissolved oxygen by site (center or skin); for example, the ground sponge originating mainly from the center is collected in the case where tough titanium metal is required while the ground sponge originating mainly from the skin is collected in the case where hard titanium metal is required.
The ground titanium metal sponge prepared in this manner is then molded into briquettes in the compression molding step (compression molding: S301) and a plurality of the briquettes are placed one upon another and welded together by the TIG welding process to yield a cylindrical electrode; thereafter the electrode is melted by vacuum arc melting, high frequency melting and the like (melting: S302) and an oxide skin on the surface is cut off to yield the product titanium ingot.
However, the smelting of titanium metal by the aforementioned Kroll process incurs an exceptionally high production cost mainly from the following causes: titanium oxide, although used as the raw material, is first converted into low-boiling titanium tetrachloride and then reduced and this procedure extends the manufacturing step; vacuum separation at high temperatures is an essential step in the manufacture of titanium metal sponge; moreover, titanium metal sponge occurring as a large lump must be crushed and ground in the manufacture of the product titanium ingot; still more, the sponge differs markedly in the concentration of dissolved oxygen between the center and the skin and the ground sponge needs to be sorted out to the one originating from the center and that from the skin depending upon the use of the product titanium ingot.
Now, several methods other than the aforementioned Kroll process have been proposed for smelting titanium metal.
For example, Sakae Takeuchi and Osamu Watanabe [J. Japan Inst. Metals, Vol. 28, No. 9, 549-554 (1964)] describe a method illustrated in FIG. 14 for producing titanium metal; a reactor consists of a graphite crucible a as an anode and a molybdenum electrode b in the center as a cathode, a mixed molten salt c which is composed of calcium chloride (CaCl2), calcium oxide (CaO) and titanium oxide (TiO2) and kept at 900-1100° C. is charged into the crucible a, the titanium oxide is electrolyzed in an inert atmosphere of argon (not shown) and the titanium ions formed (Ti4+) are deposited on the surface of molybdenum electrode b to give titanium metal d.
Another method described in WO 99/64638 is illustrated in FIG. 15: molten calcium chloride c (CaCl2) is charged into a reaction vessel, a graphite electrode a as an anode and a titanium oxide electrode b as a cathode are arranged inside the molten salt c and a voltage is applied between the graphite electrode a and the titanium oxide electrode b thereby extracting oxygen ions (O2−) from the titanium oxide cathode b and releasing the oxygen ions as carbon dioxide (CO2) and/or oxygen (O2) at the graphite anode a or reducing the titanium oxide electrode b itself to titanium metal d.
However, according to the method described in the paper of Takeuchi and Watanabe, the deposited titanium metal d is kept in continuous contact with calcium oxide of high concentration in the mixed molten salt c and this makes it difficult to produce titanium metal d of excellent toughness by controlling or lowering the concentration of dissolved oxygen in the titanium metal d being produced; moreover, titanium metal forms fine tree-shaped deposits on the surface of the molybdenum electrode b and this makes the mass production difficult. Thus, it is questionable whether the method of Takeuchi and Watanabe is suitable as a commercial method or not. On the other hand, the method described in WO 99/64638 has the following problem; the deoxidization requires a long time because oxygen is present in a small amount in the titanium metal d formed at the cathode and its diffusion in solid becomes the rate-determining step.
The inventors of this invention have conducted studies on a method and apparatus for smelting titanium metal which, unlike the Kroll process, can easily produce titanium metal without requiring the steps for vacuum separation at high temperatures and crushing and grinding of titanium metal sponge and additionally can control easily the concentration of dissolved oxygen in the product titanium metal.
Consequently, the inventors of this invention have found it possible to produce titanium metal (Ti) continuously by the thermal reduction of titanium oxide in the following manner: a molten salt consisting of calcium chloride (CaCl2) and calcium oxide (CaO) was prepared as a reaction region inside a reaction vessel, the molten salt in the reaction region was electrolyzed to generate monovalent calcium ions (Ca+) and/or calcium (Ca) thereby converting the molten salt into a strongly reducing molten salt, titanium oxide (TiO2) was supplied to the strongly reducing molten salt and the titanium oxide was reduced and the resulting titanium metal was deoxidized by the monovalent calcium ions (Ca+) and/or calcium (Ca). The inventors have further found it possible not only to produce titanium metal advantageously on a commercial scale but also to control the concentration of dissolved oxygen in titanium metal and completed this invention.
Accordingly, an object of this invention is to provide a method for smelting titanium metal which is capable of producing titanium metal commercially advantageously.
Another object of this invention is to provide a method for smelting titanium metal which is capable of producing titanium metal with a controlled concentration of dissolved oxygen commercially advantageously.
A further object of this invention is to provide an apparatus for smelting titanium metal which is capable of producing titanium metal commercially advantageously.
A still further object of this invention is to provide an apparatus for smelting titanium metal which is capable of producing titanium metal with a controlled concentration of dissolved oxygen commercially advantageously.